On the Linkage Between Asian Summer Monsoon and Tropopause Folds

Tropopause folding events are a type of troposphere-stratosphere exchange that have been linked to enhanced vertical ascent and extreme rainfall events. The latter can bring about consequences such as flooding and agricultural loss. However, much is still unknown about the mechanisms resulting in tropopause folding events. Rossby wave breaking, represented by a reversal in the potential vorticity gradient, is one such source that has been proposed in previous studies.

We investigate the climatology of tropopause folding events in order to find global hotspots of high tropopause fold frequency. Upper-level potential vorticity and mid-level vertical velocity are analyzed in three locations of interest at distinct times of frequent activity: the Asian monsoon region in summer and the North Atlantic and North Pacific oceans in winter. In addition, a set of idealized aquaplanet model runs was used to investigate the effects of tropopause folds on extreme rainfall events. It is found that tropopause folds forming in the Asian monsoon region tend to occur on the northwestern side of the upper-level anticyclone and are generated due to intensified monsoon circulation and resulting intensified subsidence rather than by Rossby wave breaking. Finally, our findings show a significant increase of precipitation downstream of the fold location and a significant decrease upstream that persists for about 1-2 days.